The invention relates to a method for shielding an electronic means against external electromagnetic radiation on one hand, and preventing the electromagnetic radiation of the electronic means from interfering with closeby apparatuses on the other hand. The electronic means may be an electronic apparatus, for example, or a unit, such as a printed circuit board assembly, in an electronic apparatus.
Electromagnetic interference (EMI) shielding of electric equipment is traditionally based on the use of either metal equipment cases or plastic cases coated with a metal layer. In addition, methods are known for manufacturing cases of a conductive plastic composite where conductive particles, such as carbon black, carbon fibres, metal fibres or metal flakes are mixed with the insulating plastic.
Disadvantages of a metal case include its weight and the high price caused by numerous machining steps. Also the coating of a plastic case with a metal layer includes many steps and is therefore uneconomic. The disadvantage of methods based on conductive plastic composites is uneven distribution of conductive particles which leaves unshielded spots in the case. Furthermore, in all the above methods it is difficult to make the EMI shielding tight because of holes and joints in the case.
At the printed circuit board (PCB) level, the EMI shielding is usually realised by a multilayer PCB (internal ground layers) or by a polymer composite containing conductive metal particles and spread by means of silk screen printing. Reference [1] discloses such a method based on a polymer composite which, however, is restricted only to shield wirings on a PCB. Since components and their coupling areas constitute a great part of the area of an electronic means, the shielding provided by this method is insufficient. Nor does a multilayer PCB shield the whole electronic means and the surface-mounted or other components on it.
Reference [2] discloses a method in which a non-conductive substrate material is coated using a chemical copper plating method. In the method, the part to be coated is submerged in a chemical bath, so usually the method cannot be applied to an electronic means because it might cause damage to the components and because the metal coating might cause a short circuit.
Reference [3] discloses a method in which the conductive shell (metal, for example) of a component is coupled to the ground plane of the equipment case. Then, separate shielding cases must be provided for the components and, in addition, the inner surface of the equipment case must be provided with a conductive layer. The extra cost of this method caused by the many steps is too high for the manufacturing of most electronic devices.
In addition, a method is known from reference [4], wherein conductive polyaniline is used to absorb electromagnetic radiation. The absorber can be used for electromagnetic absorption in the IR, visible light, UV, radar or microwave range. However, said patent document discloses no technical implementation for the EMI shielding of an electronic means.
An object of the invention is to eliminate the disadvantages of known EMI shielding methods by providing a new kind of method for the EMI shielding of an electronic means. In the method according to the invention, electromagnetic shielding is produced using a conductive film material placed near the surface of an electronic means. The conductive film is isolated from the electronic means by an insulator film. By using e.g. an intrinsically conductive polymer (ICP) such as polyaniline according to the invention, known solutions and methods can be implemented more advantageously and near to the surface of the electronic means. The method is not only an economic but also a quick way of shielding an electronic means. The method is also suitable for an electronic assembly where the coupling regions of components are unshielded because the insulator film prevents short-circuits. Another object of the invention is to provide a method for electrically connecting a conductive film material to an electronic means, advantageously to its ground potential. The inventive solution can also be used for forming capacitors and resistors for the circuits of electronic means.
The method according to the invention is characterized in that said electronic means is coated with an electrically insulating material and said insulating material is coated with an electrically conductive material. The arrangement according to the invention is characterized in that it comprises at least one electrically conductive film to shield against electromagnetic radiation and at least one electrically insulating film to insulate said electrically conductive film from electrically conductive parts in the electronic means.